Mass spectrometric study of the evaporation of Li5FeO4 as a corrosion product in the compatibility experiment of Li2O pellets with Fe-Ni-Cr alloys

The measurement of the vapor species and pressures over Li₅FeO₄ as a corrosion product in the compatibility experiment of Li₂O pellets with Fe-Ni-Cr alloys has been done in the temperature range 1200–1500 K by means of the Knudsen effusion mass spectrometric technique. The evaporation process of liquid Li₅FeO₄ was established and expressed by the reaction Li₅FeO₄(1) = LiFeO₂(s) + 4 Li(g) + O₂(g). The enthalpy of formation of Li₅FeO₄ at 298 K was derived to be 1950 kJ/mol from the third-law treatment. Although a congruent process was suggested for the sublimation, it could not be confirmed in detail.     

Temperature distribution in Li2O pellets under neutron irradiation

The center temperature of Li₂O pellets under neutron irradiation was measured using an encapsuled pin. The surface temperatures of the pellets and the pellet/cladding heat transfer coefficients were estimated on the basis of the well-known conductivity integral.     

Reaction of molybdenum and molybdenum-base alloy with sintered Li2O pellets

The reaction of sintered lithium oxide (Li₂O) pellets with molybdenum and molybdenum-base alloy TZM has been studied in the temperature range 800–1100 °C for a constant reaction period of 100 h under a dynamic vacuum. The reaction proceeded measurably at about 950°C and appreciably above 1000°C. Metallographic and X-ray diffraction analyses indicated that a reaction product scale was formed at the interface and the scale consisted mainly of Li₄MoO₅. TZM showed slightly higher reactivity compared to molybdenum.     

Corrosion behavior of Fe-Ni-Cr alloys in the molten salt of LiCl-Li2O at high temperature

At Korea Atomic Energy Research Institute (KAERI), we investigated the corrosion behavior of a series of Fe-Cr-Ni alloys with different chromium contents in molten LiCl and molten LiCl-25wt%Li₂O mixture at temperatures ranging from 923 to 1123 K. In molten LiCl, dense protective scale of LiCrO₂ grows outwardly while corrosion is accelerated by addition of Li₂O to LiCl. The basic fluxing of Cr₂O₃ by Li₂O would be the cause of accelerated corrosion. Because of low oxygen solubility and very high Li₂O activity in the molten LiCl-Li₂O mixture, Cr is preferentially corroded while Ni remains stable and thus, corrosion rate of the alloys in molten LiCl-Li₂O mixture increases with an increase in Cr content.     

Reaction of several iron and nickel based alloys with sintered Li2O Pellets

The reaction of type 316 stainless steel, Incoloy 800, Hastelloy X-R, Inconel 600 and pure Ni with sintered Li₂O pellets has been studied between 800 and 1100°C under dynamic vacuum. The reaction products were analyzed by means of metallographic, microprobe and X-ray diffraction methods. The reactions proceeded measurably between 800 and 950°C and appreciably at 1000°C, being greatest with Incoloy 800 and least with Hastelloy X-R. Among the primary alloy constituents, chromium was exclusively attacked by lithium and oxygen diffusing from the Li₂O into the alloys to form LiCrO₂. This phase grew into a reaction zone (subscale) of uniform thickness beneath the surface of each alloy. Preferential growth of LiCrO₂ along the grain boundaries was observed only in the case of Inconel 600 below 950°C. On the other hand, iron diffused toward the Li₂O pellets to form volatile Li₅FeO₄. However, any reaction product associated with Ni was not detected and Ni metal was little attacked by the Li₂O pellet over the whole range of reaction temperature.     

Thermodynamics of the Li2O fusion reactor breeder

Lithium oxide is one of the candidate tritium breeders for a fusion reactor. A computational survey, using experimentally measured activity coefficients for LiOH in Li₂O, was made of the thermodynamic behavior of Li₂O as a fusion reactor breeder blanket material. All significant species involved in establishing an equilibrium state, including those generated from the effects of H-T exchange, are included in the analysis. Oxygen activity, particularly at values less than approximately 10^?15, is a variable that was found to strongly influence the balance of the molar quantities and activities of various species. These relationships are important in assessing the HT-HTO balance in the gas phase, the enhancement of tritium release by protium purging, the vaporization of LiOH/LiOT and Li, and the effects of elemental lithium and LiT in the condensed phase.     

Diffusion of tritium in single crystal Li2O

The release of tritium from neutron irradiated spherical samples of single crystal Li₂O was measured by isothermal annealing experiments. The release is shown to be controlled by diffusion of tritium in the solid under appropriate experimental conditions. Deviations from solely diffusion controlled release were observed when traces of water were present in the He-purge gas used in the experiments. The diffusivity of tritium in single crystal Li₂O is given by In(D/cm₂s^?1) = ? (5.93 ± 0.48) ? (81.73 ± 4.24)10³J/RTfor 850 K < T < 1200 K. (R = 8.314 JK^?1mol^?1).     

Determination of the order of surface reactions in Li2O

An analysis of the different surface reactions taking place in Li₂O was performed in order to determine whether adsorption and desorption of tritium are first or second order reactions. Data from BEATRIX-II Phase I and CRITIC-I were used as basis for calculations.It was found that only second order adsorption/desorption on the surface of Li₂O can predict the tritium behavior observed experimentally.     

Compatibility between several heat resistant alloys and sintered Li2O in static helium gas environment

The reaction of sintered Li₂O discs with several commercial heat resistant alloys has been investigated under the conditions of 3.3 × 10⁴Pa ($\text{1}\text{3}\text{atm}$) static He gas atmosphere in the temperature range of 500 and 750° C. Reaction products were identified by X-ray diffraction analysis to be two phases of Li₅FeO₄ and LiCrO₂. The former was dominant below 650° C and the latter was dominant above 650° C. The activation energies of the reaction were determined by the Arrhenius plots for weight changes and sub-scale thickness measurements. The reactivity of the four Fe-Ni-Cr alloys decreased according to the order of Incoloy 800, 316 SS, Hastelloy X-R and Inconel 600. Grain boundary penetration was observed above 500° C for Incoloy 800, 550° C for 316 SS and 600° C for Inconel 600. There was no grain boundary penetration in Hastelloy X-R.     

Mass spectrometric investigation of the vaporization of li2tio3(s)

The vaporization of Li₂TiO₃(s) has been investigated by the mass spectrometric Knudsen effusion method. Partial pressures of Li(g), LiO(g), Li₂O(g), Li₃O(g) and O₂(g) over Li₂TiO₃(s) have been obtained in the temperature range 1180–1628 K. When the vaporization of Li₂TiO₃(s) proceeds, the content of Li₂O in the Li₂TiO₃(s) sample decreases. The phase of the sample is a disordered Li₂TiO₃ solid solution above 1486 K. The enthalpies of formation and the atomization energies for LiO(g) and Li₃O(g) have been evaluated from the partial pressures to be ΔH^o_f0(LiO, g) = 65.4 ± 17.4 kJ/mol, ΔH^o_f0(Li₃O, g) = − 207.5 ± 56.6 kJ/mol, D^o₀(LiO) = 340.5 ± 17.4 kJ/mol and D^o₀(Li₃O) = 931.6 ± 56.6 kJ/mol, respectively.     

Short-lived fission product release from UO2: Annular versus solid pellets

We have irradiated, in a “dual sweep” test assembly, one element containing solid UO₂ pellets with four, $\text{1 × 1}$ mm surface slots and one element containing annular UO₂ pellets with a central hole two mm in diameter. Data are reported for a linear power range of 17–57 kW/m; thermocouples monitored operating temperatures. He-2%H₂ carrier gas swept short-lived fission products from both elements past independent spectrometers for identification and measurement. The releases from the solid and annular fuel were power-dependent from 17–57 kW/m, with the annular fuel release up to eight times greater than that for solid fuel. The annular fuel also showed a larger released iodine inventory in fuel and sheath areas with access to the carrier gas.     

Synthesis and sintering of Li2SnO3

As one of candidates for solid blanket materials of fusion reactors, lithium stannate Li₂SnO₃ was synthesized by precipitation of a complex, lithium hexahydroxystannate Li₂Sn(OH)₆, and its thermal decomposition. In order to get single-phase precipitates Li₂Sn(OH)₆, aqueous solution SnCl₄ had to be added slowly to LiOH aqueous solution. Decomposition of Li₂Sn(OH)₆ to Li₂SnO₃ proceeded in two steps, abrupt release of about two molecules of water around 200° C and gradual release of remaining water above 260° C. The grains of Li₂SnO₃ obtained at 800° C were porous, consisting of small primary particles of about 0.2 μm size. For the pellets prepared from these porous grains under 20 and 100 MPa, the density saturated to a value of 1.8 and 2.4 g/cm³ within two days at 1000° C, corresponding to the porosity of 64 and 52%, respectively. The porous nature of the original Li₂SnO₃ grains was found to be kept even after 1000° C treatment for a week. For the pellet prepared by ground Li₂SnO₃ under 200 MPa, remarkable sintering was observed at 1000° C; density increased rapidly and became close to theoretical density (7% porosity) after four days. The product Li₂SnO₃ synthesized by the present process has advantages in terms of manufacturing, handling and possibly tritium recovery of the blanket.     

Surface desorption and bulk diffusion models of tritium release from Li2TiO3 and Li2ZrO3 pebbles

The release of tritium from Li₂TiO₃ and Li₂ZrO₃ pebbles, in batch experiments, is studied by means of temperature programmed desorption. Data reduction focuses on the analysis of the non-oxidized and oxidized tritium components in terms of release limited by diffusion from the bulk of ceramic grains, or by first or second order surface desorption. By analytical and numerical methods the in-furnace tritium release is deconvoluted from the ionization chamber transfer functions, for which a semi-empirical form is established. The release from Li₂TiO₃ follows second order desorption kinetics, requiring a temperature for a residence time of 1 day (T_1dRes) of 620 K, and 603 K, of the non-oxidized, and the oxidized components, respectively. The release from Li₂ZrO₃ appears as limited by either diffusion from the bulk of the ceramic grains, or by first order surface desorption, the first possibility being the more probable. The respective values of T_1dRes for the non-oxidized component are 661 K, according to the first order surface desorption model, and 735 K within the bulk diffusion limited model.     

Characterization of ThO2-UO2 pellets made by co-precipitation process

The co-precipitation technique renders an excellent route to obtain a homogeneous mixture of ThO₂ and UO₂ powders. In this process, after the nitrate solutions of Th and U are mixed in the intended ratio, oxalic acid is added for co-precipitation. The precipitate is then dried and calcined to get a solid solution of ThO₂ and UO₂. In this study, ThO₂-30%UO₂ and ThO₂-50%UO₂ (% in weight) powders were characterized in terms of particle size, particle shape, surface area, phase content, O/M ratio etc. The pellets obtained by sintering these powders were characterized with the help of optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The XRD data for ThO₂-30%UO₂ and ThO₂-50%UO₂ pellets showed the presence of a small amount of U₃O₈ phase besides fluorite phase. The grain size of ThO₂-30%UO₂ and ThO₂-50%UO₂ was found to be 5.7 and 4.5 μm, respectively.     

Lithium and tritium diffusion in lithium oxide (Li2O), a molecular dynamics simulation

Molecular dynamics simulations of Li₂O were performed in the microcanonical ensemble at several different temperatures in order to study the lithium diffusion process, and a preliminary exploration of the diffusion of tritium was performed. Different Li/O molar ratios were used to investigate the role of non-stoichiometry in the Li diffusion processes. The mechanism of lithium diffusion as a function of temperature is proposed based on the analysis of our simulations and a model is proposed to explain the overall behaviour of the lithium diffusion coefficient as a function of temperature. Our simulations suggest what is the role of hydrogen in the tritium release from breeder ceramic materials.     

An experimental study on linear differential scattering coefficients of the radioactive compounds UO2(C2H3O2)2 · 2H2O and Th(NO3)4 · 5H2O at 60 keV

The linear differential scattering coefficients at 60 keV have been measured for UO₂(C₂H₃O₂)₂ · 2H₂O (uranyl-acetate) and Th(NO₃)₄ · 5H₂O (thorium-nitrate) radioactive compounds at seven angles ranging from 60° to 120° at intervals 10°. The obtained results have been compared with relativistic and non-relativistic theoretical values.     

Preliminary study of spent UO2 fuel corrosion in the presence of bentonite

The corrosion of spent UO₂ fuel in the presence of a dilute suspension (1.5%) of bentonite in synthetic groundwater has been studied. No significant changes in the uranium concentrations no indications of increased corrosion due to changes in solution chemistry, or due to sorption was found when bentonite was introduced to the system. The measured uranium concentrations were (3 ± 2) × 10⁻⁶M in the presence as well as in the absence of bentonite. The concentrations of plutonium and cationic fission products in the aqueous phase were lowered considerably, by up to two orders of magnitude in the case of plutonium due to sorption onto the bentonite.     

Fabrication of Li2TiO3 pebbles by water-based sol-gel method

Fabrication of Li₂TiO₃ pebbles by wet process received great attention for their convenience to realize mass production and good performances. Li₂TiO₃ pebbles with about 1.4 mm in diameter were prepared by a water-based sol-gel method using Ti(C₄H₉O)₄ and LiNO₃ as raw materials. This process is simple and has not been reported previously. Phase analysis, thermal analysis and morphological observation were carried out with the pebbles. The experimental results showed that a large amount of pores were presented on the surface of the pebbles and the density of the pebbles was sensitive to the sintering temperature. The pebbles sintered at 1200 °C for 10 h reached a density of ∼68%T.D. The shape of the pebbles was favorable with an average sphericity of about 1.08.